Stereotactic radiosurgery eligibility and selection bias in the treatment of glioblastoma multiforme

Several single institution studies have shown a survival advantage when a stereotactic radiosurgery (SRS) boost followed fractionated external beam radiation (FracRT) in the treatment of glioblastoma (GBM). RTOG 93-05 employed SRS before FracRT and demonstrated no survival benefit. We examined the effect of SRS eligibility before and after FracRT on patient outcome in a group of patients treated with conventional therapy without SRS. From 1998 to 2008, 106 patients with GBM treated definitively at the University of Utah were divided into groups based on eligibility for SRS: ineligible ("Never''), eligible before FracRT ("All Pre''), eligible before FracRT only ("Pre Only''), or eligible before and after FracRT ("Always''). Overall (OS) and progression-free survival (PFS) based on SRS eligibility was assessed. Eleven patients were alive at the time of analysis with a median follow-up of 42.3 months. Median OS for groups "All Pre'' (n = 29), "Always'' (n = 17), "Pre Only'' (n = 12), and "Never'' (n = 77) were 13.6, 13.6, 12.4, and 9.2 months, respectively. Of the 29 patients in group "All Pre,'' 12 (41.4%) were ineligible for SRS following FracRT. PFS did not significantly differ between groups. SRS for GBM can only be of benefit to selected patients with minimal focal postoperative disease. Following FracRT, over a third of initially SRS-eligible patients demonstrated more extensive disease in our experience. It is possible inclusion of such patients in a series of SRS for GBM could mask a benefit in remaining patients. No significant difference in OS or PFS based on SRS-eligibility status was found.